timing(MainPipe): fix critical path from Dir error to GrantBuffer

Consider the following timing path:

The `error` signal returned by Directory is used by forwarding snoop to
determine whether to allocate an MSHR. If there is an error, a SnpResp
is returned directly; otherwise, an MSHR needs to be allocated. This
signal also affects the opcode sent to GrantBuffer. GrantBuffer uses
whether the opcode is HintAck to decide whether to enqueue.

However, the opcode is, either the opcode pre-set by MSHR in MSHR
requests, or `sink_req_s3.bits.opcode` generated in s3. It can be
determined simply by using `mshr_req_s3` because when `valid` output to
GrantBuffer is false, we do not need an accurate opcode.

Author: linjuanZ

src/main/scala/coupledL2/tl2chi/MainPipe.scala

@@ -663,6 +663,7 @@ class MainPipe(implicit p: Parameters) extends TL2CHIL2Module with HasCHIOpcodes
   txdat_s3.bits.task := source_req_s3
   txdat_s3.bits.data.data := data_s3
   d_s3.bits.task := source_req_s3
+  d_s3.bits.opcode := Mux(mshr_req_s3, req_s3.opcode, sink_req_s3.bits.opcode)
   d_s3.bits.data.data := data_s3
 
   when (task_s3.valid) {